The present invention relates to processes and apparatus for treating a textile web with a treatment liquid containing a treatment medium, such as a dye bath, in which the textile web is continuously guided through a trough containing the treatment liquid, and subsequently passed through a pair of squeezing rollers.
One such process and corresponding apparatus are known from the reference "Melliand Textilberichte" 1/1989, pages 46 to 52, particularly page 52, FIG. 23. The trough and the pair of squeezing rollers together form a conventional foulard. In the known embodiment, a system for "concentration regulation," shown schematically, can be seen in the trough of the foulard, which is supposed to be able to take place "locally (edge/center) and/or laterally (run-off/tailing)." However, there is no information in the reference to indicate what is to be done with the concentration values that are determined. The pair of squeezing rollers of the foulard is entirely neutral, i.e. it is shown without any reference to a control mechanism.
The present invention begins from consideration of problems in the continuous dyeing of textile webs on a foulard. In this connection, particularly in the case of substantive and reactive dyes, the effect occurs that water, as the solution and transport medium, is absorbed more slowly or more rapidly by the web as it passes through the trough of the foulard, relative to the dye components in the dye bath. If the web absorbs water more rapidly, the dye bath loses water and the concentration of dye becomes higher. This means that the depth of shade increases, i.e. that the beginning of the web is clearly dyed a lighter color than the end of the dye lot in question.
A typical example for this case is dyeing viscose with reactive dyes. Initially, viscose swells very much and entrains a lot of water from the dye bath.
However, the reverse case also occurs, that a certain textile web absorbs more dye from the dye bath. This means that the beginning of the dye lot is dyed with a greater depth of shade.
When dyeing with reactive dyes, another cause for a nonuniform dye result over the length of the web is the tendency of reactive dyes to hydrolyze. Hydrolyzed reactive dye is no longer available for the actual dyeing process, and can therefore lead to concentration changes of reactive reactive dyes.
Color changes over the length of the web are also referred to as "tailing." As a rule, they proceed according to a positive or negative e function and end in a state of equilibrium; after this equilibrium is reached, no further changes take place. Changes over the length of the web can be influenced not only by the properties of the fiber material, but also by physical properties such as strong water absorption of the web and swelling processes.
Although the causes of tailing are essentially known today, the problem continues to exist in practice and the initial lengths of some dye lots still have to be sold as seconds or have to be rejected entirely. The solution approaches tried until now have not been very successful. These solution approaches were, for example, to use dyes with a low affinity and/or hydrolysis constant, to reduce the temperature in the dye trough, or also to minimize the bath content in the dye trough.
In many cases, all these solution approaches reach clear limits, particularly in the important case of dyeing viscose with reactive dyes, which was mentioned, where starting lengths on the order of several tens of meters show color deviations which result in spoilage. Since shorter and shorter lengths of dye lots are being demanded today (down to as little as a hundred meters), there clearly is an urgent need to address this problem. The several tens of meters of starting length which cannot be used represent too high a proportion of damaged goods.